1. Field of the Invention
This invention relates to a small-sized motor provided with a worm reduction gear, and more particularly to a small-sized motor wherein a worm is press-fitted onto a motor shaft and a turning force of the motor is transmitted to a load through a worm wheel being in meshing engagement with this worm, for example, one effectively used in a power window motor for a motor vehicle.
2. Related Art Statement
As a power window motor for a motor vehicle, conventionally there has been used such a small-sized motor provided with a worm reduction gear that an output from the motor is reduced in speed through a worm reduction gear; a motor shaft thereof is lengthened; and a worm is press-fitted onto the intermediate portion of the lengthened motor shaft.
As a small-sized motor of this type, for example, there is one disclosed in Japanese Utility Model Laid-Open No. 1-133545, which is constructed as shown in FIG. 3.
That is, this small-sized motor with the worm reduction gear is provided with a motor section and a reduction gear section, which are connected in series with each other, wherein a motor shaft 1 is racked over the whole length of the motor section and the reduction gear section, being rotatably supported by a bearing 2 on the motor section's side, a bearing 3 on the reduction gear section's side and a bearing 4 in the intermediate portion. A worm 5, an armature and a commutator are press-fitted onto the motor shaft 1 and fixed thereto. The worm 5 is in meshing engagement with a worm wheel 6 in the reduction gear section, whereby a reduced turning force of the motor shaft 1 is transmitted to an output shaft 7 fixed to the worm wheel 6.
The bearing 4 in the intermediate portion is constructed to be supported in a motor case by a bearing holder 8 formed separately from the motor case and a gear case and be centered through the bearing holder 8. This bearing holder 8 additionally functions as a brush holder stay. Furthermore, to improve the workability in assembling the worm 5 onto the motor shaft 1, the bearing 4 in the intermediate portion is spaced at a satisfactory distance L from the worm 5.
However, the small-sized motor with the worm reduction gear having the above-described arrangement presents the following problems.
(1) The bearing 4 in the intermediate portion is supported by the motor case through the bearing holder 8 formed separately from the motor case and the gear case, so that the number of parts and the number of assembling processes are increased.
(2) Moreover, to maintain the precise coaxial relations between the bearings 2, 3 and 4 disposed at three positions for supporting the motor shaft 1, it is necessary to strictly control the working accuracy and the assembling accuracy of the bearing holder 8, motor case, gear case and the like.
(3) If the coaxial preciseness of the bearings at the three positions for supporting the motor shaft 1 are low, then the meshing accuracy between the worm 5 and the worm wheel 6 is decreased, whereby, in the motor such as the motor for a power window used under a heavy load, there is a risk that the worm wheel 6 made of a resin material may be broken.
(4) The bearing 4 in the intermediate portion is spaced far from the worm 5, whereby the motor shaft 1 is easily flexed, so that the meshing accuracy between the worm 5 and the worm wheel 6 may be decreased.
(5) During the work of press-fitting the worm 5 onto the motor shaft 1, first, the bearing holder 8 is coupled onto the motor shaft 1 in a hanging state, whereby a jig for holding this bearing holder 8 is required, so that not only the work becomes complicated but also it becomes difficult to manufacture a small-sized motor of high quality.